There have been known a fixing device which includes a cylindrical fixing film, a nip plate slidingly contacting an inner peripheral surface of the fixing film, a pressing roller having the fixing film interposed between the nip plate and the pressing roller, a heater disposed at an inside of the fixing film, a stay supporting the nip plate with surrounding the heater and a guide rib provided to the stay and guiding the inner peripheral surface of the fixing film. In this related-art, a nip portion formed between the nip plate and the pressing roller is heated by the heater, so that a toner image is heat-fixed on a sheet when the sheet passes through the nip portion.